Many types of semiconductor memory have been developed. Some memory types are volatile and lose their contents when power supply is removed. Some memory types are non-volatile and retain the information stored in the memory even after power supply has been removed. For this reason, the non-volatile memory has been widely used in memory cards and other applications. One type of non-volatile memory is flash memory, which stores charge in a charge storage region of a memory cell.
The flash memory device may be classified into two types. One of the types is a NAND type flash memory device and the other type is an OR type flash memory device.
In the field of NAND flash memories, as a result of the rapid advance in reduction of device size for the purpose of cost savings through enhancement of bit density, cell size has nearly reached a physical limit. Therefore, a stacked nonvolatile memory formed by three-dimensionally stacking memory cells is attracting attention as a means of attaining higher bit density.
This Discussion of the Background section is provided for background information only. The statements in this Discussion of the Background are not an admission that the subject matter disclosed in this Discussion of the Background section constitute prior art to the present disclosure, and no part of this Discussion of the Background section may be used as an admission that any part of this application, including this Discussion of the Background section, constitutes prior art to the present disclosure.